Compounds that inhibit the biosynthesis of L-asparagine and thymidylate are antitumor agents. The mechanisms of action of certain known antitumor drugs which may act as specific inhibitors of L-asparagine synthetase and orotidine 51-phosphate decarboxylase (the enzyme responsible for catalyzing the last step in the biosynthesis of uridylate, which is needed to produce thymidylate) are being investigated. The mechanisms of these two enzymes are being studied; chemical model studies are being used to clarify the enzyme chemistry. Based on the determined mechanisms of these enzymes, specific irreversible inactivators are being designed and synthesized, and will be tested in vitro and in vivo as antitumor agents. These inactivators also will be used to determine the amino acid sequence catalytically active residue at the active sites of the enzymes.